Depends on your definition of free space. There are a bunch of existing recorded Tivo Suggestions still there
That would be free space.

It takes a while for TiVo to leave what you like and don't like, it does this based on what you've recorded and what you given thumbs up or down too.
I also think that it compares your viewing habits to other peoples viewing habits, so lets say you had a season pass for Friends - it would looks at what other people with SP for Friends commonly also had SP for and Suggest that.

If you'd like to have the odd Buffy episode on had to watch when your bored do set up a season pass for it, reduce the number of episodes it hangs on to down to one or two, and make sure it is down the bottom of your Season Pass list (the most recently added SP is down the bottom by default).

Have you added a bigger HDD to it yet?